This application is based on Japanese Patent Application No. 2000-091611 filed in Japan on Mar. 29, 2000, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method for recording an image onto a thermoreversible recording medium capable of multicolor display, and a recording apparatus therefor.
2. Description of the Related Art
In recent years, reversible thermosensitive recording media capable of repeating recording and erasing of an image have received attention from the viewpoints of visualization of electronic information and resource savings, and some of them have become commercially available. As typical examples thereof, high polymer/low polymer mixed types and leuco dye/long-chain alkyl developer mixed types are known.
Since recording of a larger amount of information is required hereafter, there has been demanded a reversible thermosensitive recording medium capable of full-color and high-precision recording. However, the display colors are limited to colors of opacity and transparency for the high polymer/low polymer mixed type. Whereas, there also occurs color development of only a single color or two colors for the leuco dye/long-chain alkyl developer mixed type.
As the materials effective for a recording medium capable of multicolor recording and erasing, there have been known cholesteric liquid crystal materials with a molecular weight of about 1000 in JP-A-11-24027. The cholesteric liquid crystal materials are characterized by showing selective reflection for only a light of a specific wavelength within the liquid crystal phase temperature region, and having a selective reflection wavelength that changes with temperature. Further, for the cholesteric liquid crystal material, the selective reflection state for a specific wavelength can be fixed by quenching from the liquid crystal phase temperature region to the solid phase temperature region, which becomes a feature effective as the material for the thermoreversible recording medium.
However, the study by the present inventors has proved that the cholesteric liquid crystal material changes in color, while varying in the length of time required for recording and erasing (response time) depending upon temperature. Therefore, the following problem may occur. That is, in the case of recording through the application of heat for a fixed time, changing into a prescribed color is not completed because of an insufficient quantity of heat for recording at a temperature requiring a long response time. Further, for recording at a temperature requiring a short response time, the material is applied with heat more than required, resulting in wastes of time and energy.
Under such circumstances, it is therefore an object of the present invention to provide a recording method and a recording apparatus on a thermoreversible recording medium which promotes effective utilization of applied energy, and enables high-quality recording over the entire display color region.
The present inventors have conducted a close study for attaining the foregoing object, and have obtained the following findings. (1) The cholesteric liquid crystal material has a different response time for each selective reflection wavelength, so that an optimum energy application time is required for obtaining a desired recording color. Especially, since the emission color precision increases with a decrease in heating temperature, a certain degree of application time becomes necessary. (2) The application time in the foregoing item (1) can be adjusted by the pulse width and/or pulse number of the applied energy.
With the recording method achieved based on the foregoing findings in accordance with the present invention, the energy application time is extended with a decrease in heating temperature for a thermoreversible recording medium capable of multicolor display, provided with a thermosensitive recording layer containing a cholesteric liquid crystal material.
The recording apparatus in accordance with the present invention includes an energy application means for extending the energy application time with a decrease in heating temperature for a thermoreversible recording medium capable of multicolor display, provided with a thermosensitive recording layer containing a cholesteric liquid crystal material.
The cholesteric liquid crystal material is comprised of not less than one of liquid crystalline compound that exhibits a cholesteric phase at a temperature range higher than room temperature, and reflects light in a visible wavelength region corresponding to the temperature, and is fixed still in the reflection state by quenching from the temperature. Examples of such a material include the materials as described in Adv. Mater. 1997, 9, No. 14, pp.1102-1104, and liquid crystal materials having other chemical structures, and having the same properties are also acceptable. To the liquid crystal materials, other liquid crystalline compounds and non-liquid crystalline compounds may also be added.
By varying the energy application time on the thermosensitive recording layer containing the cholesteric liquid crystal material described above, it became possible to perform high-quality recording with high contrast over the entire display color region, and thus to utilize the applied energy effectively. The energy application time can be adjusted by the time width and/or number of a pulse.
The energy application can be accomplished by a heat generating unit such as a thermal head, or by means of laser irradiation. When heating is conducted through the laser irradiation, a photothermal converter is preferably provided in the recording medium.